A toxin is a substance that causes damage to biological systems by chemical means. The term is usually reserved for substances that are life-threatening in small quantities. (Other substances, including water, can be lethal in large quantities, but are not usually called poisons.)

Orally administered toxins are also called poisons, especially if intentionally administered by a human. Animal toxins that are delivered subcutaneously (e.g. by sting or bite) are also called venom. (In normal usage, a poisonous organism is one that is harmful to consume, but a venomous organism uses poison to defend itself while still alive. A single organism can be both.)

The onset of symptoms of poisoning may be rapid and swiftly lead to illness or death. Examples are poisoning due to inhalation of hydrogen cyanide or injection of potassium chloride. This is called acute poisoning.

A poison may also take effect slowly. This is known as chronic poisoning and is most common for poisons that bioaccumulate. Examples of these types of poisons are mercury, lead, and asbestos.